A. Field of the Invention
The invention relates to retarded acid-in-oil emulsions and to methods of treating an acid soluble subterranean formation with such an emulsion.
B. Description of the Prior Art
Calcareous formations such as limestone, dolomite or other calcareous-magnesium formations which are at least partially soluble in acid are often treated with an acid to increase the permeability thereof. Such treatments are particularly valuable in stimulating wells to enhance the recovery of valuable formation fluids such as petroleum or gas.
One difficulty in the past which has been experienced in this field is the rapid, spontaneous action of the acid on the formation. The rate at which the acid reacts with the formation is a function of various factors including acid concentration, temperature, fluid viscosity, the particular type of calcareous material, and the surface area to acid volume ratio. In acid treatments of wells the acid can be pumped only a certain distance into the formation before the acid becomes spent. The time required for the acid to become spent is generally referred to as the reaction time. To more efficiently treat certain formations it is desirous to increase the reaction time of the acid. This is true for instance when the acid solution is to be employed to also fracture the formation.
Basically three approaches have been taken to extend the reaction time of acidizing compositions.
One approach is to employ an acid-in-oil emulsion. An early emulsion patent is de Groote, U.S. Pat. No. 1,922,154. de Groote, however, appears to have been more concerned about protecting the casing in the wellbore from corrosion rather than in retarding the action of the acid on the formation itself. Cardwell et al., U.S. Pat. No. 2,802,531 employed an emulsion to keep a proppant suspended in a fracturing method, but does teach the aqueous phase could be an acid. Cardwell et al. mention as suitable emulsifiers, long chain amides (col. 3, line 28) -- but not ethanol amides, and hydrochlorides of long chain amines (col. 3, line 45). Jones, U.S. Pat. No. 3,044,549, employed acid and oil as a mixture, but apparently not as an emulsion (see col. 2, line 49 and column 5, line 19). Knight, U.S. Pat. No. 3,353,603 employed an emulsion using an alcohol sulfate or dodecylbenzenesulfonic acid as a surfactant/retarder. Fast et al., U.S. Pat. No. 3,681,240 is similar to Knight, except that Fast et al. employed an amine salt of dodecylbenzenesulfonic acid rather than the free acid. Crowe, U.S. Pat. No. 3,779,916 employed an acid-in-oil emulsion containing a saturated hydrocarbon sulfonate and an alkylarylsulfonic acid or water soluble salt thereof. Crowe, U.S. Pat. No. 3,917,535 obtained increased penetration of the acid into the formation by controlling the fluid loss using an oil-in-acid emulsion. Crowe, U.S. Pat. No. 3,962,102 employs a retarded emulsion containing a C.sub.8 to C.sub.18 primary amine.
A second approach to retarding the action of the acid is to pretreat the formation with a composition which renders the formation oil wet. Art representative of this technique includes Knox et al., U.S. Pat. No. 3,343,602 and Bombardieri, U.S. Pat. No. 3,434,545. Somewhat related are Thompson et al., U.S. Pat. No. 2,908,643, which discloses treating limestone with a C.sub.8-12 aliphatic amine to render the limestone preferentially oil wet, and Blair, Jr., U.S. Pat. No. 2,331,594 which teaches a similar treatment for silica, but the patents do not apply their methods in the context of acidizing.
The third technique referred to involves including a retarding surfactant directly in the acid, which acid remains substantially non-emulsified. Knox, U.S. Pat. No. 3,319,714, for example, employs an anionic surfactant in the acid. Crowe, in U.S. Pat. Nos. 3,917,536 and 3,962,101 teaches use of a C.sub.8-18 primary amine and, optionally, a quaternary amine. Reed, U.S. Pat. No. 2,370,421 teaches an acidizing composition containing a mixture of sulfonic acids, but appears to be directed more toward altering the surface tension of the acid rather than retarding the acid.
Of the foregoing, the retarded acid emulsions of Crowe, U.S. Pat. Nos. 3,779,916 and 3,962,102 are believed most pertinent. The emulsion of U.S. Pat. No. 3,779,916 has proved quite successful commercially, particularly when used with moderately concentrated acid, e.g., about 15% HCl and less. The degree of retardation obtained with the emulsion of U.S. Pat. No. 3,779,916 using concentrated acid, e.g., above 25% HCl, at high temperatures, e.g., above 150.degree. F, however, is somewhat less than desirable for widespread commercial application. The emulsion of U.S. Pat. No. 3,962,101 offers greater retardation, particularly with highly concentrated acid at elevated temperatures than does the emulsion of U.S. Pat. No. 3,779,916, but has as a drawback the fact that the emulsion does not always break upon substantial spending of the acid thereby making well clean-up somewhat difficult.